I've realised the Samsung GX-1S
does offer stop down metering with a Pentax-M series lens. You just have to enable two things in the Custom Setting menu -
1. "Using aperture ring" - Enable shutter release when lens aperture is set at other than A. (Why would you
not want this on the whole time?)
2. "FI with S lens used" - Enable focus indicator when screw mount lens in use. Lens is recognised even when no lens on camera. (This also provides the visual focus confirm indicator even with manual focus lenses, which I thought only worked with A series and newer on the GX-1S. It seems really accurate too.)
You also have to go into "AE-L button on M exposure" in the Custom menu and set it to Tv Shift.
These all enabled, the AE-L (ie exposure lock) button enables stop down metering when an M lens is set to an aperture other than wide open, like the "green button" on my K10D. You can still tweak the shutter speed up or down with the dial. Even better, whilst I find the green button's position and ergonomics a bit awkward on the K10D, the AE-L button on the GX-1S is much easier to locate and have your thumb on ready, so as a result the GX-1S is easier to use with M (and K) lenses than my K10D. I'm unlikely to get any more K or M lenses as I'm focusing on M42 now (90% Takumar) but at least I can use the ones I still have more easily now.
In the meantime I tried out my Flektogon 35/2.4 for the first time on the GX-1S today. Yeh I know it's not a Pentax lens, but it's pretty impressive, and focuses less than 0.2m...